1. Field of the Invention
The present invention relates to an erasable image forming material and a method for manufacturing the same.
2. Description of the Related Art
Protection of forests is important for protecting the global environment and for suppressing the greenhouse effect caused by CO2. It has been a great problem to efficiently utilize paper resources in order to keep the amount of additional felling of trees to a minimum and to maintain the balance between the amount of tree felling and the regeneration of forests including reforestation.
Reuse of presently available paper resources consists of recycling, by which paper pulp, after a de-inking step for extracting image forming materials, are made into low-quality paper again to be used according to respective purposes. However, several problems have been pointed out regarding this reuse method, including the high cost of the de-inking step and the additional environmental pollution caused by the disposal of waste liquid.
In contrast, reuse of hardcopies by, for example, an eraser for pencils and correction liquid for ball-point pens has been practically used. While reused paper sheets are used for the same purpose plural times by preventing paper quality from being degraded as much as possible, recycled paper sheets may be used for other purposes, although the quality of the paper is reduced. Accordingly, reuse and recycling involve different concepts from one another.
Reuse is more important in terms of protection of paper resources, and the amount of additionally required paper resources can be suppressed to a minimum level when appropriate reuse is performed before recycling. For example, rewritable paper has been proposed in recent years as special paper aiming at reuse of hardcopy paper sheets. One hundred or more use cycles is possible by using rewritable paper technology so long as slight damage to the paper such as wrinkling and creasing is not an objection, and efficiency of use of paper resources may be remarkably improved.
However, rewritable paper is not suitable for recycling since it is special paper, although reuse is possible. It is difficult to apply rewritable paper to recording technologies other than thermal recording.
The inventors of the present invention have noticed that a color is developed when the interaction between a color former and a developer increases, while the decolorization occurs when the interaction decreases. The inventors have developed an image forming material, which enables a decolorized state of the image forming material to be stably maintained at around room temperature by adding a decolorizing agent to a composition containing the color former and developer and which fixes the decolorized state for a long time by treating with heat and a solvent, an image decolorizing process, and an image decolorizing apparatus. The inventors have proposed this novel technology as a paper reuse technology in place of currently available technologies.
The image forming material has high stability of colored and decolorized states of the image as well as being quite safe, and is applicable to uses such as electrophotographic toners, liquid inks, ink ribbons and any writing instruments. In addition, the material has the advantage of the possibility of large-scale decolorizing treatment, which has been impossible in the conventional art.
The inventors have also found that cellulose as a constituent element of paper has a function of the decolorizing agent, and have proposed that decolorization is possible by treating with heat and a solvent even in the image forming material that contains no decolorizing agent in restricted uses using paper as a recording medium.
For example, it has been disclosed that a clear image can be formed by using an image forming material containing the color former, the developer and a binder resin, while the image can be favorably decolorized. Decolorization is possible in this image forming material since the developer is transferred to the paper sheet by heating at a predetermined temperature and the color former becomes insensitive to the developer.
However, the image forming material itself showed weak color because only a part of the color former in the image forming material turned into a colored state.
While an alternative method in which color density is increased by adding the color former in excess may be devised, the excess color former that does not contribute to color development is wasted, while the excess compound adversely affects the properties of the image forming material. For example, storage stability and mechanical strength are reduced since the additive amount of low-molecular-weight components different from major components (matrix) increases in the color forming material.